Metal uptake and root morphological changes for two varieties of Salix integra under cadmium stress Differences in Uptake and Tolerance to Cadm ium in Var ieties of Sa lix in tegra Variations in Lead Accumulation and Tolerance in Different Varieties of Salix integra and in Situ Distribution of Elements in Leaves under Pb Stress Effect of flooding stress on growth and photosynthesis characteristics of Salix integra Tolerance of different varieties of Salix integra to high zinc stress